1. Field of the Invention
The invention relates to a thin film transistor, a method for manufacturing the same, a display device, and a method for manufacturing the same.
2. Description of the Related Art
Thin film transistors (TFTs) are widely used for liquid crystal display devices, organic EL display devices, and the like.
Amorphous silicon TFTs used for large-size liquid crystal display devices have a mobility of approximately 1 cm2/Vs and they can be formed uniformly with a large area at low cost. Further improvements in size and resolution are required these days. Furthermore, an active matrix organic EL display devices which require a large driving current have been developed, and a novel active material of low cost, high uniformity, high reliability, and high mobility is required.
In the development circumstances mentioned above, oxide semiconductors receive attention as a material usable for the channel layer of TFT in these days.
For example, TFTs which have a transparent conductive oxide thin film containing ZnO as a main component for the channel layer are developed actively. The thin film mentioned above can be film-formed with a large area at a relatively low temperature, and achieve a higher mobility than amorphous silicon. For example, JP-A 2004-103957 (Kokai) discloses a TFT using In—Ga—Zn—O-based amorphous oxide. Since the thin film mentioned above can be film-formed at a low temperature and is transparent in the visible range, it is considered that a flexible and transparent TFT can be formed on a substrate of plastic or film. Furthermore, approximately ten times as high field-effect mobility as amorphous silicon is obtained.
On the other hand, it is reported that, for example, the electric conductivity of oxide semiconductor changes with oxygen concentration during sputtering film-formation (for example, see Applied Physics Letters, 90, 192101 (2007)). Thus, in regard to oxide semiconductors, electrical characteristics are very sensitive to the concentration of contained oxygen, and the oxygen concentration changes due to heat treatment to cause deteriorated characteristics, for example. This is a major factor that prevents TFTs using an oxide semiconductor from their practical use.